Diode-pumped lasers involve elongated semiconductor diodes which lase and producing unfocused radiation output when excited by an electrical field established thereacross. The lasing portions of the semiconductors are positioned near a laser gain medium such as a crystal (with a regular and repeated atomic lattice) or a glass (without a regular atomic lattice). Since most side-pumped diode lasers use crystals, the term "crystal" is occasionally used below for convenience, to refer to a preferred type of laser gain medium.
The unfocused laser energy from the semiconductors is directed into the crystal either directly or via one or more lenses. When "pumped" by the laser energy from the semiconductors, the energy excitation levels (sometimes referred to as population inversions) build up within the atomic structure of the crystal to an extent which causes the crystal to lase. The medium, which is usually elongated and provided with a mirror or other reflective coating at each end, focuses the laser energy along its main axis. The laser beam exits through one of the mirrors, which is made to be only partially reflective.
The term "laser head" is often used to refer to the assembly which comprises the gain medium, the lasing diode(s), the mirror(s), and any mounting devices which hold those components in their proper positions and alignment. A laser head can also include other components that are directly attached to the diode and crystal assembly, including possibly a housing and any necessary heat sinks. Typically, a laser head will include one or more electrical ports for the leads that supply the electrical energy from an external power source to the diodes.
The diode(s) used to excite or "pump" a laser crystal can be mounted at one end of an elongated crystal to create an end-pumped laser, or along one or more sides of the crystal to create a side-pumped laser. End-pumped lasers are limited in power, since only one diode or a small number of diodes can be mounted in close proximity to the end of the crystal. Side-pumped lasers can be more powerful, since a much larger number of diodes can be mounted along the length of the crystal to pump energy into it. Alternately, the radiant emission can be conveyed from the diodes to the crystal by means of one or more fiber-optic cables, but fiber-optic couplings suffer from reduced power, greater manufacturing expense, and other problems.
For more information on diode-pumped lasers see, e.g., W. Koechner, Solid State Laser Engineering (Springer-Verlag, New York, 1988), the article "Diode-Pumped Solid-State Lasers Have Become A Mainstream Technology" by G. T. Forrest in Laser Focus/Electro-Optics. November 1987, pp. 62-74, the article "Advances in Diode Laser Pumps" by W. Streifer et al in IEEE Journal of Quantum Electronics 24 (6): 883-984 (June 1988), and various patents such as U.S. Pat. Nos. 4,864,584, 4,805,177 and 4,901,324 (Martin; assigned to Laser Diode Products, Inc., the predecessor company to the Applicant's assignee, Laser Diodes, Inc).
As is well known to those skilled in the art, one of the primary problems in operating any laser which utilizes a crystal lasing medium involves controlling and specifically limiting the temperature of the active elements including the laser diode pump sources on the gain medium or crystal. Two commonly used compositions for laser crystals are Nd:YAG (neodymium ions in a matrix of yttrium-aluminum-garnet) or Nd:YLF (neodymium ions in yttrium-lithium-fluoride). Such crystals are preferably operated at temperatures in the range of about 0.degree. to about 30.degree. C., from freezing to moderate temperatures. However, laser excitation can generate high temperatures, including localized "hot spots," which can interfere with the power and precision of the laser beam, and power and heat related problems reduce the strength and/or intensity of the beam. Precision-related problems also exist and are generally referred to as "noise" and can include variations in the wavelength or power, scattering and dispersion of the beam, and comparable problems.
In diode-pumped lasers, the diodes employed generate heat (including hot spots) and must be cooled. If not reduced, high temperatures (i.e., increased vibrations of the atomic lattices in the semiconductor material) can reduce operating efficiency, and can permanently damage or destroy the semiconductor materials of the diodes. High temperatures can also cause the diodes and crystal to become misaligned.
The magnitude and importance of the cooling problem can be seen in perspective by considering the efficiencies of diode-pumped lasers (usually measured in terms of the amount of power carried in the laser beam, divided by the total electrical power consumed by the laser equipment). Typically most diode-pumped lasers operate in the range of about 1% to 5% efficiency. Therefore to generate a laser beam carrying one watt of energy, a diode-pumped laser will have to dissipate as much as 100 watts or more of input energy and most of that energy must be dissipated as heat.
Furthermore, many lasers which cannot otherwise cope adequately with the problem of cooling must be operated only in a pulsed mode; i.e., their output is limited to short bursts of laser energy. Between pulses, such lasers must be deactivated so they can cool. However, it is often preferable and desirable in many applications to be able to operate lasers in the continuous wave (CW) mode.
Some types of lasers use a cooling fluid (such as water) which is pumped through one or more passages in the metallic structures used to mount the heat-generating components of the laser. However, such fluid-coolant systems require additional components, including a pumping system and a heat exchange system to cool the fluid. Those components add bulk, weight, and expense to the assembly, and can lead to various operating problems such as leakage of the coolant fluid. Water cooled lasers are also limited as to the environment in which they can be used.
Some diode-pumped lasers use devices called "thermoelectric coolers" (TEC's) to cool the crystals and the diodes to desired operating temperature ranges. In general, a TEC is a sandwich configuration, with two ceramic plates surrounding or on opposite sides of a plurality of "Peltier devices." The Peltier devices, which are usually int he shape of columns or cubes which separate the two plates, are made of a special alloy. When a suitable current is passed through a Peltier pellet, one end of the pellet becomes cold (the temperature can drop below 0.degree. C.) and the other end becomes hot. The laser crystal and the diodes are mounted on or near the cold side, while the hot side of the TEC is attached to a heat sink which has means to dissipate the heat, often with the aid of a fan which flows air across fins on the heat sink.
Various types of heat sinks have been developed in the prior art for dissipating the heat generated by diode-pumped lasers. Conventional heat sinks are made of a heat-conductive material such as copper or aluminum, which usually contains fins to increase the exposed surface area. A cooling fan is often provided to blow air against the fins. Conventional heat-sinks of this nature are shown in, e.g., FIGS. 1 and 4 of U.S. Pat. No. 4,805,177 (Martin et al 1989; assigned to applicant's assignee).
A diode configuration which offers improved heat control is shown in U.S. Pat. No. 4,864,584, which is also assigned to Applicant's assignee. That patent discloses a structure with arrays of numerous very small elongated diodes which are closely packed together, see FIGS. 15 and 16 and Column 7 of this patent. In that arrangement, the heat generated by the numerous small diodes is distributed relatively evenly over the area on which they are mounted. Since the heat is distributed more evenly across the diode mount, hot spots, which can adversely effect operation, are minimized.
Despite those improvements, there remains a need to provide even more precise control over the temperatures of the diodes and crystals in side-pumped lasers. In particular, there is a need for a laser head assembly that provides independent temperature control over the diodes and the gain medium, since the optimal lasing temperature of a certain type of diode may be different than the optimal lasing temperature of a certain type of crystal. In some situations, it may be desirable to operate the diodes and the crystal at temperature differentials from each other, sometimes exceeding 20.degree. C. In addition, since temperature can affect the wavelength and the frequency of the laser light, precise temperature control, if available, may be used to "tune" a laser diode or crystal to a preferred frequency.